Which event describes a change where evolution has happened? An allele frequency changes in a population. A white-eyed fly is born into a population of brown-eyed flies. A species of tree dies out. A school of fish moves to a new environment.

A white-eyed fly is born into a population of brown-eyed flies.  

Explanation:

In the example, a new mutation (i.e., white-eyes) has appeared in the progeny. In the case that this new mutation has adaptive value, it will be transmitted across generations and therefore will increase its frequency in the population

to this one it would be receptive field